1. Field of the Invention
The present invention generally relates to an improved heated tool, usually wedge shaped, for use in welding machines used to join thermoplastic sheets together. More particularly, the invention relates to a heated tool which is designed to advantageously condition the sheet material adjacent to and along the edge of the actual weld.
2. Description of the Prior Art
The welding together of overlapped thermoplastic sheets by heating the upper surface of the lower sheet and the lower surface of the upper (overlapping) sheet, above their melting point, and then applying pressure so that the two melted surfaces join and fuse into a weld is a well established and documented practice. One of the earliest patents in the U.S. is U.S. Pat. No. 4,146,419, Inventor Wolfgang Neidhart, in which many features of currently used welding machines are described. This type of welding machine is widely used in the pollution control industry which lines landfills and other waste containment facilities with chemically resistant and durable thermoplastic sheet liners known as geomembranes.
The most widely used method of heating the surfaces of the sheets to be welded is by dragging a heated block of metal between the overlapped sheets. This block is usually heated by imbedded electrical resistance heaters, but can be heated by induction heating. This block is usually made in the form of a wedge, the wedge tapering to its thin end just ahead of the device which squeezes the two sheets together, usually by means of a set of opposing pinch rollers. It is common to produce two parallel weld tracks to facilitate air pressure testing of the weld seam.
The heating can also be achieved by a jet of hot air (or gas) being blown through a nozzle inserted between the overlapped sheets. This method is less favored because the low specific heat of air and the impracticality of blowing high volumes of air, means the only way of providing sufficient energy to melt the sheet surfaces is to raise the temperature of the air to a level which is damaging to the thermoplastic molecules, causing unwanted degradation.
Detailed descriptions of so called Hot Wedge welding can be found in: xe2x80x9cHot Wedge Fusion Welding of HDPE Geomembranesxe2x80x9d by Gary M Kolbasuk Published in xe2x80x9cThe Seaming Of Geosyntheticsxe2x80x9d edited by R. M. Koerner, published by Elsevier Applied Science ISBN 1 85166 4831 and; xe2x80x9cConsistent Wedge Welderxe2x80x9d by Fred Struve in xe2x80x9cGeosynthetic Liner Systems: Innovations, Concerns, and Designxe2x80x9d Published by Industrial Fabrics Association International ISBN 0 935803 01 7.
Until the present the welding parameters of concern have been the temperature of the heated wedge, the speed of welding, the force between the pinch rolls, and in addition the question of using a radiused shape for the heating surface, as opposed to a flat shape has been considered.
Long term durability tests of the welds have consistently shown that failures, especially of the low load stress crack growth type, occur at or very near to, the edges of the welds. One of the issues of concern which has been raised is the fact that the surface of the sheet which is dragged over, and is in intimate contact with, the heated surface of the wedge is subject to relatively high temperatures whereas the sheet immediately adjacent to it, only a few thousandth of an inch away, is only subject to ambient temperature. This severe temperature gradient at the edge of the weld is likely to cause significant morphological differences along the edge of a weld, which contribute to this being a locus of weakness. This subject is investigated and discussed in xe2x80x9cThe Influence of the Welding Parameters for HDPE Geomembranesxe2x80x9d by A. L. Rollin et al in a paper presented at the 6th International Conference on Geosynthetics, Atlanta, March, 1998.
In a primary embodiment, the invention provides for an improved heated wedge, which is wider, for some distance along its length, than the width of the desired weld track. If the length of the wedge in the direction of the weld track is considered to be 100%, then by increasing the width of the wedge over say 50% of its length, the sheet on either side of where the weld track will be, is subjected to heating by the wedge for half the time that the actual weld area is subjected to heating by the wedge. The temperature gradient at the edge of the weld track is thereby reduced. The amount of additional width and percentage of the wedge length chosen for xe2x80x9ctaperingxe2x80x9d the amount of heat supplied to the sheet along the edge of the weld track is variable and subject to design and testing for best results.